1. Field of the Invention
The present invention relates to windmill airfoils or rotors and, more particularly, to setting pitch on a windmill airfoil.
2. Description of the Prior Art
Alternative energy sourcing is a popular area of study and practice given the increasing cost and finite reserves of conventional energy sources. Harnessing wind, the realm of the age-old windmill, is one popular alternative energy resource. The technology in windmill apparatus, however, has greatly improved the design and efficiency of older conventional windmills. Typically, modern windmill apparatus comprises two or more windmill rotor blades having aerodynamic characteristics similar to aircraft propellers. Such aerodynamic rotor blades are commonly referred to in the industry as thick airfoils. As in aircraft use, the pitch of the rotor blade (the angle of the blade about its longitudinal axis generally measured at a given chord of the airfoil relative to the air flow direction) determines air flow and, in the case of windmills, the amount of force generated by the air flow. The desired rotor blade pitch of a given windmill is determined by the location of the windmill, expected air flow characteristics, altitude, temperature, season, power or torque desired and speed of rotation desired, among other factors.
Failure to set appropriate pitch generally results in inefficient or even non-operating windmills. For example, a difference of as little as 3.5.degree. pitch can cause a decrease in energy efficiency of over 46% in the same wind environment. Moreover, certain airfoils peak at a given wind speed and efficiency decreases at greater wind speeds. Accordingly, pitch needs to be adjusted to compensate for the higher wind conditions. In addition, all of the rotor blades on one windmill preferably should have identical pitch to optimize performance and efficiency of the windmill and to decrease the potential for overload and instable conditions (commonly referred to as yaw loads) that occur when two or more rotor blades do not have the same pitch.
Rotor blade pitch is conventionally adjusted and set in the field with the windmill apparatus already erected. Methods for setting and adjusting rotor blade pitch on windmills typically comprise holding the rotor blades in close proximity to the rotor hub or fastening point of the windmill, rotating the rotor blade about its longitudinal axis until the desired pitch is achieved and fastening the rotor blade to the rotor hub. Achieving the desired pitch in these prior art methods requires measuring the arc offset from the last known pitch angle and/or eye-bailing the desired pitch from a convenient vantage point. This method often leads to at least slight variations in the pitch angle of the rotor blades on a rotor hub due to the lack of an objective measuring standard. In addition, because windmill rotor blades have diameters from 7 to 20 meters and are relatively heavy, it is usually necessary to suspend the rotor blade by a crane or other lifting apparatus to hold the rotor blade in position until the pitch can be determined and the rotor blade is fastened to the rotor hub.
In view of the deficiencies and inefficiencies of the prior art, it would be desirable to have a method for setting pitch on a windmill rotor blade or airfoil which is safer, more accurate and more convenient than prior art methods.